1. Field of the Invention
The present invention relates to a display panel module and a method of forming the same, and more particularly to a display panel module having an improved connection structure between a display panel and a circuit board and a method of forming the same.
2. Description of the Related Art
Flat display panels of various types such as liquid crystal display panels, plasma display panels and electroluminescent display panels have been well-known to persons skilled in the above-described field. Such a display panel comprises an insulating substrate which has electrode terminals. The display panel forms a display panel module in combination with a flexible printed circuit board for driving the display panel. The display panel and the circuit board are electrically connected to each other through the following connection structure.
The display panel has electrode terminals. The circuit board has lead terminals. The display panel and the circuit board sandwich an anisotropically conductive sheet, so that the electrode terminals and the lead terminals are spatially separated by the anisotropically conductive sheet but electrically connected to each other through the sheet. Japanese Patent No. 2964730 discloses a plasma display panel having such a connection structure.
FIG. 1 is a fragmentary cross sectional elevation view of a plasma display panel module with a conventional connection structure. The plasma display panel module comprises a plasma display panel and a flexible printed circuit board 5. The plasma display panel has a supporting substrate 1 which has a surface, on which electrode terminals 2 and insulating barrier layers 12 are provided, wherein adjacent two of the electrode terminals 2 are separated by the insulating barrier layer 12. The electrode terminals 2 has a lower height than the insulating barrier layer 12. The flexible printed circuit board 5 has a surface having an alignment of lead terminals 6 which are positioned in correspondence with the electrode terminals 2.
The plasma display panel and the circuit board 5 sandwich an anisotropically conductive film 4 which has conductive particles 15, so that the electrode terminals 2 and the corresponding lead terminals 6 are spatially separated by the anisotropically conductive film 4 but electrically connected to each other through the conductive particles 15 in the anisotropically conductive film 4. Since adjacent two of the conductive particles 15 are separated, the anisotropically conductive film 4 is conductive in a film thickness direction but insulative in an in-plane direction. For this reason, adjacent two of the electrode terminals 2 are electrically isolated and other adjacent two of the lead terminals 6 are also isolated.
The electrode terminals 2 are made of silver. The insulating baffler layers 12 are provided for preventing that the electrical isolation between the adjacent two of the electrode terminals 2 is deteriorated by silver migration upon a heat treatment which is carried out for a thermal compression bonding process for bonding the substrate 1 and the circuit board 5 through the film 4. Assuming that the above insulating barrier layers 12 are not provided, then the silver migration may be caused via a migration path of a binder for the material of the film 4, wherein the migration path is present at a gap between the adjacent two of the electrode terminals 2.
Advanced displays have narrowed alignment-pitches of the electrode terminals and the lead terminals. Narrowing such alignment-pitches causes such an inter-electrode migration easily. The provision of the above insulating barrier layers 12 is effective to suppress the migration. The inter-electrode migration causes a short circuit between the electrode terminals. Such a migration is gradually caused during a long term operation of the display. The reason for the short circuit between the electrode terminals is not only the migration but also other factors on the manufacturing processes.
One of the other reasons is a difference in thermal expansion coefficient between the substrate 1 and the flexible printed circuit board 5. As described above, the substrate 1 and the flexible printed circuit board 5 are bonded through the anisotropically conductive film 4 by the thermal compression bonding process. The flexible printed circuit board 5 is made of an organic material such as polyimide which has a thermal extension coefficient of about 26xc3x9710xe2x88x926/xc2x0 C. The supporting substrate 1 is made of a transparent glass material which has a thermal extension coefficient of about 5xc3x9710xe2x88x926/xc2x0 C. Namely, the flexible printed circuit board 5 has a higher thermal extension coefficient by approximate five times than the supporting substrate 1.
Upon receipt of the heat in the thermal compression bonding process, the flexible printed circuit board 5 shows a larger expansion than the supporting substrate 1. FIG. 2 is a fragmentary perspective view of the display panel module after the thermal compression bonding process, wherein the electrode terminals of the substrate and the lead terminals of the circuit board have become misaligned due to those difference in thermal expansion coefficient. Before the thermal compression bonding process, the electrode terminals of the substrate and the lead terminals of the circuit board are just aligned. The thermal compression bonding process causes the substrate and the circuit board to show different thermal expansions, whereby the electrode terminals of the substrate and the lead terminals of the circuit board become different in pitch, resulting in a misalignment between them. The degree of the misalignment is larger at the opposite end portions than the center portion. A large relative displacement between the electrode terminals and the lead terminals causes a short circuit, wherein adjacent two of the electrode terminals are electrically connected to each other through the lead terminal having both edges in contact with edges of the adjacent two electrode terminals. Narrowing the terminal pitch reduces such a critical relative displacement, which causes the short circuit. Increasing the number of the terminals increases the relative displacement at the opposite end portions. The advanced display module has the serious problem with the short circuit formation.
Japanese laid-open patent publication No. 5-249479 discloses the following conventional technique for avoiding such a short circuit formation between the terminals. FIG. 3 is a schematic perspective view of a flexible printed circuit board with a single slit at a center. The flexible printed circuit board 5 has a single slit 14 at a center position. The flexible printed circuit board 5 also has a surface on which plural lead terminals 6 are aligned. The single slit 14 extends in parallel to a longitudinal direction of the lead terminals 6 and in perpendicular to an alignment direction of the lead terminals 6. The slit 14 reduces a thermal expansion of the circuit board 5 in the alignment direction. The slit 14 makes it necessary to do plural times alignment processes, thereby dropping the productivity of the display panel module. The formation of the slit 14 causes the increase of the manufacturing cost. The slit 14 weakens the mechanical strength of the circuit board 5, thereby reducing the reliability of the display panel module.
In the above circumstances, it had been required to develop a novel display panel module and method of forming the same free from the above problem.
Accordingly, it is an object of the present invention to provide a novel display panel module free from the above problems.
It is a further object of the present invention to provide a novel bonding structure between a display panel and a circuit board free from the above problems.
It is a still further object of the present invention to provide a novel method of forming a display panel module free from the above problems.
It is yet a further object of the present invention to provide a novel method of bonding a display panel and a circuit board free from the above problems.
A first aspect of the present invention is a display panel module comprising: a substrate; a circuit board; electrode terminals aligned in a first direction over a surface of the substrate, each of the electrode terminals extending in a second direction perpendicular to the first direction; lead terminals aligned in the first direction over a confronting surface of the circuit board to the substrate; at least an anisotropically conductive film sandwiched between the electrode terminals and the lead terminals; and a plurality of first electrically insulating walls provided on the substrate and positioned in at least selected plural ones of gaps between selected ones of the electrode terminals, and the selected ones of the electrode terminals being positioned on opposite side regions of the module, and the opposite side regions are distanced in the first direction and separated by a center region, wherein the first electrically insulating walls have a first height, which is higher than a first total height of the electrode terminals and the anisotropically conductive film.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.